As the demand for lithium-ion batteries increases, the requirement for the technology is higher as well, such that the batteries manufactured can have greater energy density and better electrochemical properties to meet the market demand. Here, enhancing the capacity of the lithium-ion batteries, their cycling performance, and the electrochemical properties at high and low temperatures is the trend for current and future technical improvements of the batteries.
Adding additives to the electrolyte to improve the properties of lithium-ion secondary batteries has been a research emphasis during the recent years. Since the SEI membrane (formed in the reactions between active matters and the electrolyte during the initial charging process) can suppress solvent from co-intercalating into the graphite and has no negative effects on the transmission of Li+, the characteristics of the additives are important factors in affecting the properties of a battery. Thus they have become an emphasis and focus for research. Currently, quite a few property-improving additives optimize the SEI membrane. For example, U.S. Patent 2004091786 discloses that by adding propylene sulfite (PS), a stable SEI membrane can be formed on the surface of the carbon negative electrode. This membrane will not break at high temperatures, blocks the reaction between the electrolyte and the negative electrode, restricts the creation of gases, and improves properties at high temperatures. A. Naji has discovered that by adding ethyl sulfite (ES) in a propylene carbonate (PC) electrolyte system, the co-intercalation phenomenon of PC in graphite is suppressed (see Electrochim. 2000, (145):1893). Chinese Patent CN1260850 discloses that by adding ES, the cycle life of batteries is improved. In addition, inorganic membrane-forming additives including carbon dioxide and sulfur dioxide can improve certain aspects of the properties of batteries. Many different researches show that by adding vinylene carbonate (VC), a passivation film can be formed on the surface of electrodes. The reduction potential of VC is higher than that of the solvents including ethylene carbonatediethyl carbonate (EC), PC, diethyl carbonate (DEC), dimethyl carbonate (DMC), etc. Thus VC can be reduced first on a carbon negative electrode and forms stable SEI membrane, thereby improving the stability of a battery in the cycle process, especially the stability at high temperatures (see J. Electrochem. Soc., 2001, 148:1341˜1345). U.S. Patent 2004062995 discloses that by adding VC, a surface film can be formed, thereby preventing the dissolution of electrolyte and improving the cycle life.
Although by merely adding the above additives, the properties of the batteries can be improved to a certain degree, but the effect is not very distinctive. Often, a certain property is improved and the others are not affected. Since the additives are often costly, and some of the additives are used in a large amount when it is used by itself, the costs are greatly increased. As the market and customers increase their demand for high capacity and usability properties in different operating conditions, the industry is pushed for better products. Therefore, it is distinctively important to find an additive with good overall properties, or a combination of additives which can improve various properties of batteries. In addition, it is important that the cost of the additives must be within an acceptable range.